Vitamin D deficiency occurs among breastfed infants in all parts of the U.S. but is particularly common among infants with dark skin pigmentation and infants living in the northern parts of the U.S. where synthesis of vitamin D in the skin is diminished or absent during winter. The prevalence of vitamin D deficiency, including its severest form, rickets, is on the rise, probably because of the increase in breastfeeding. Vitamin D supplementation can prevent vitamin D deficiency. Current recommendations are to provide a daily supplement of 200 IU of vitamin D. However, it is not established that this dose prevents vitamin D deficiency reliably. Nor is it known how effective higher doses are and what the optimal dose is for infants. The objective of the proposed study is to establish in definitive fashion the effectiveness, or lack thereof, of three vitamin D dosages in the prevention of vitamin D deficiency in breastfed infants. The study will also examine an array of functional indicators of vitamin D sufficiency in the hope of establishing the optimal amount of vitamin D that infants need in order to remain in good health. Infants to whom this applies include infants with dark skin regardless of season and Caucasian infants living at northern latitudes during winter. Breastfed infants will be recruited at 1 month of age and will be assigned at random to receive a daily supplement of either 200 lU/day or 400 lU/day or 600 lU/day from 1 to 9 months of age. They will receive no supplemental formula until age 9 months. This will be a double blind study. Infants will be seen monthly and blood will be drawn by heel stick on seven occasions for determination of 25-OHD and of functional indicators of vitamin D status. Ultraviolet exposure will be estimated also. Bone mineral content will be determined once (in March or April, i.e., at the end of winter). The other endpoints will be measured on multiple occasions. The primary endpoint will be vitamin D status, i.e., plasma concentration of 25-OHD, with deficiency defined as concentration <15 ng/ml. We hypothesize that regular administration of 400 IU and 600 IU of vitamin D each day will prevent vitamin D deficiency reliably but that administration of 200 lU/day will be less than completely effective. Functional indicators of vitamin status will increase with increasing doses of supplemental vitamin D. The study has considerable public health importance because of the importance of preventing vitamin D deficiency not only in its severe form, rickets, but also in its milder forms.